Non-isolated DC-DC converters are widely used in modern electronic equipment. These converters are directly built right next to the load, and are also referred to as point-of-load (POL) power supplies. With increasing demand to miniaturize voltage regulators for computers and telecommunication products, high power density POL converter topologies are highly desirable. The inductor in a conventional buck converter occupies a large portion of the buck converter's space. It is difficult to reduce the size and weight of the inductor. For example, in current POL power modules, inductors occupy most of the substrate space such that they are the most significant barrier to increasing power density.
Increasing the switching frequency is a common and simple way to reduce the inductance required for a POL buck converter. A drawback of this approach is that the power loss will be increased and the switches will overheat. Also, increasing the switching frequency beyond tens of MHz in a non-integrated POL buck converter is impractical since a very small duty cycle is needed to provide very low output voltage (usually 0.5-1.6V) for modern microprocessors. A very small duty cycle limits the maximum switching frequency. The single phase three-level buck converter shown in FIG. 1A is a potential candidate for replacing the conventional buck converter in high power density POL converters because the operating frequency can be doubled. More capacitors need to be added to such a single phase three-level buck converter to further increase the operating frequency (e.g., a four-level buck converter), but the extra capacitors complicate the operation of the converter.
When high output current is needed, multi-phase converters may be used to improve the efficiency of the entire system. For a conventional multi-phase buck converter such as two-phase buck converter shown in FIG. 1B, the output current is not evenly shared by each phase because of component tolerance and various parasitic components on the printed circuit board (PCB) tracks. Uneven output current sharing will cause uneven thermal dissipation and will decrease efficiency of the converter. Thus, auxiliary circuits and components are needed to share the output current evenly, which increases the cost of the converters and also complicates their control.
In a POL application, most loads require very low output voltage ranging from 0.5V to 3.3V. At the same time, higher output voltages are also required, such as by USB ports (5V), and for dynamic voltage scaling. However, the voltage gain of the converters is small because of the duty cycle limitation. Accordingly, the maximum output voltage of the converters is limited. This problem exists in most three-level DC-DC converters.